Everybody knows what communication is; the conversion of thoughts and images into ideas that can be spread among many people in a way that they can easily understand. For example, if I am in a situation where I need help with some particular task, such as jumping my car when it will not start, my primary tool for acquiring said help is communication. Communication requires both language and rhetoric; the right words in the right context, presented in the right way, in order to be truly effective.
Given the above information, the most important aspect of communication is its ability to bring very different people together and unite them towards a goal. Although humans are capable of much intelligence, we are incapable of doing incredible things all by ourselves. For example, you, the reader, could not be reading my blog if it wasn't for the existence of the internet, a feat that required several people collaborating together to come into existence, and the knowledge of which no one person holds by themselves.
All of the greatest aspects of the modern world, from medicine and pharmaceuticals, to rich, vast history of our species, are so complex that "encyclopedic" knowledge of any topic in existence is practically impossible. In order to maintain our greatness as a species and continue growing over time, we must continue specializing in tasks that are useful for the endeavors of humanity. Thus, in order for modern society to be successful, everyone must learn to communicate efficiently and effectively, so that a greater collaboration of ideas can maintain the intellectual infrastructure of the 21st century.